This invention relates to processes for conducting chemical reactions in the presence of a molten inorganic salt.
A number of chemical reactions can be conducted in the presence of a molten salt. Among these are, for example, coal gasification reactions, propene oxidation, pyrolysis of kraft lignin, preparation of mixed phosphates, electrodeposition of aluminum from aluminum chloride, deoxidation of niobium and titanium, destruction of cyanides, titanium production, synthesis of 2-arylpropionic esters, reductions of nitrogen gas, oxidations of methane to methanol, electrodeposition of titanium onto alumina, and silicon nitride, spent oxide fuel reduction processes, palladium-catalyzed Trost-Tsuji C-C coupling reactions, various electrochemical reactions and among many others. The molten salt can perform various functions, depending on the particular reaction being performed. Thus, the molten salt can act as a reagent, a catalyst or even a solvent, depending on the particular reaction system. In some cases, the salt may perform more than one of these functions.
Notable classes of chemical reactions that can be conducted in the presence of a molten salt are oxidation reactions of organic compounds, i.e., combustion reactions. Combustion reactions are perhaps more important than any other class of industrial chemical reactions, as they provide tremendous quantities of heat to drive, or to produce steam which is then used to drive, the turbines that generate most of the world""s electrical power. The abatement and control of environmental pollution caused by combustion products are a major world problem and the focus of global efforts to reduce xe2x80x9cgreenhouse gasxe2x80x9d emissions that ultimately contribute to global warming.
Of particular significance is the reduction of NOx and SOx that are associated with the combustion of natural gas at high temperatures near 1200xc2x0 C. Although modern systems use lean premixed combustion gases to reduce temperatures, the combustion temperatures are still high enough to promote NOx formation. In addition, the high temperatures also require that dilution air be added to meet turbine temperature requirements. Because the production of NOx depends exponentially on temperature, it is desirable to develop a method for combustion of natural gas and other vaporizable fuels, such as light diesel fuel, at lower temperature, preferably less than 1000xc2x0 C. It is also desirable to simultaneously capture in-situ any residual sulfur species present in the fuel to eliminate SOx formation as well.
It has been proposed to accomplish this by conducting the oxidation reactions in the presence of certain molten salts, which permit lower temperature combustion to take place. See, for example, U.S. Pat. Nos. 3,647,358, 3,642,583 and 4,246,255. Molten salts have been used to selectively catalyze various oxidation reactions. Most non-charged materials are soluble in molten salts. It is believed that the solute acquires an electrostatic orientation in the melt, reducing the energy required to initiate and sustain chemical reactions. Under these conditions, the solute, when exposed to oxygen, will oxidize at temperatures lower than those normally required for oxidation while maintaining high oxidation efficiencies. Molten salt catalysts have been used to selectively oxidize various fuels at reduced temperatures due to the fuel""s solubility in the molten salt and the lower required activation energies.
Although molten salt technology provides substantial potential advantages, practical problems have largely prevented its implementation into commercial processes. The main problem is that it is has been difficult to get sufficient mass transfer between a reagent stream and a bed of molten salt to operate the process efficiently. Gaseous reactants that are bubbled through a molten salt bed tend to form large bubbles that have relatively low interfacial surface areas (i.e. between the gas and molten salt phases). As mass transfer rates, and thus reaction rates, will depend on interfacial surface area, this low surface area tends to result in low conversions. Conversions in principle can be improved by making the interfacial surface area larger (such as by making smaller bubbles) or by making the residence time greater, but neither of these approaches has been found to be practical, especially for large scale operations. These approaches result in excessive energy requirements or excessive capital expenses for equipment necessary to contain a large bed of molten salt.
Thus, it would be desirable to provide an improved method for conducting chemical reactions in the presence of a molten salt.
In one aspect, this invention is a method for using a molten salt to carry out a chemical reaction, comprising passing a chemical reagent or mixture thereof through a fluidized bed of support particles that support at least one molten salt, under conditions such that said reagents or mixture of chemical reagents react in the presence of the molten salt.
In a second aspect, this invention is a method for the low-temperature oxidation of an organic compound, comprising passing a mixture of the organic compound and an oxygen source through a fluidized bed of support particles that support at least one molten salt that catalyzes the oxidation of the organic compound, under conditions such that the organic compound is oxidized in the presence of the molten salt.
In a third aspect, this invention is a process for using a molten salt to carry out a chemical reaction of a gas, comprising:
providing a molten salt supported on fine particles in a fluidized bed; and
reacting the gas in the presence of the molten salt; wherein the molten salt catalyzes a reaction of or reacts with the gas.
In a fourth aspect, this invention is a method for low-temperature oxidation of gaseous species, comprising the steps of:
providing a supported molten salt catalyst in a fluidized bed; and
contacting the catalyst with at least one gaseous species to be oxidized.